The present invention relates to a display optical system that provides to an observer an enlarged image of an original image formed on an image-forming element such as a liquid crystal display element. More particularly, the invention relates to a display optical system suitable for an image display apparatus such as a head mounted display (HMD).
An HMD is desired to provide a wider viewing angle for a higher realistic sensation and have a reduced apparatus size. Further, the HMD mounted on a head (in front of eyes) of an observer is particularly desired to be thin in view of weight balance on the head.
A simple solution to meet the demand for a wider viewing angle would be to use a large image-forming element that forms the original image, which, however, is not preferable because such a large image-forming element would invite a cost increase. To widen the viewing angle using a small image-forming element, an image-forming optical system that forms an intermediate real image of the original image once and provides an enlarged virtual image of the intermediate real image is more suitable than an image-forming optical system that forms no intermediate real image and provides an enlarged virtual image of the original image to an observer. The problem with the image-forming optical system that forms the intermediate image once is that the number of constituent elements is large and an optical path of the optical system is long, meaning the system tends to be bulky and heavy.
An HMD designed to achieve a wider viewing angle and smaller size is disclosed, for example, in Japanese Patent Laid-Open No. 2003-149593. In this HMD, a shuttling optical path is formed using a returning reflective surface and decentered reflective surfaces through which the light passes before and after the returning reflective surface, and thereby a long optical path necessary for forming one intermediate image is accommodated within a small optical system.
However, even this HMD disclosed in Japanese Patent Laid-Open No. 2003-149593 has not achieved a sufficiently wide viewing angle and a sufficient size reduction.